We recommend the Samsung Galaxy Tab S2 9.7", especially if you will be scanning photo access cards since it has a good quality camera. Amazon link

Other than OTG support, there are no other specific requirements, although if you are going to be scanning QR codes on Photo Access Cards, then it is good to use a tablet with a higher resolution camera and that works OK in low light like the Galaxy Tab S2.

Also note that while the tablet is connected to the camera, you can not charge most tablets (although some do allow this), so if the event is a long one you might need a second tablet that can be charging while you use the first one. We also offer a solution that allows the tablet to be charged while it is connected.